1. Field of the Invention
The present disclosure generally relates to an image forming apparatus having an image density detector and a toner concentration detector, and more particularly to an image forming apparatus which adjusts a image density and toner concentration with an image density detector and toner concentration detector.
2. Discussion of the Background
An image forming apparatus includes a photoconductive member, a charger to charge the photoconductive member, an optical writing unit to write a latent image on the charged photoconductive member, a developing unit to develop the latent image formed on the photoconductive member as a toner image with an effect of developing bias voltage, and a transfer unit to transfer the toner image formed on the photoconductive member to a recording sheet.
Such image forming apparatus may further include a toner amount detector, which can be used to detect a relationship between the developing bias voltage and toner amount, and a controller, which controls conditions of the charger, optical writing unit, developing unit, and transfer unit based on a result detected by the toner amount detector.
In such image forming apparatus, the relationship between the developing bias voltage and toner amount can be expressed by a linear function based on information detected by the toner amount detector.
The image forming apparatus may further include a condition storing unit, which stores a plurality of patterns of controlling parameters consisting of parameters for a charger, optical writing unit, developing unit, and transfer unit in advance. Based on the developing bias voltage, the controller can select a pattern of controlling parameters from the condition storing unit, and control the charger, optical writing unit, developing unit, and transfer unit with the controlling parameters selected from the condition storing unit.
In such an image forming apparatus, a two component developer, having toners and carriers, may be used in the developing unit to develop an image on a recording sheet (e.g., transfer sheet).
If an image area ratio produced on the recording sheet becomes smaller, a toner amount to be adhered on the recording sheet becomes smaller, wherein the image area is an area where toner actually adheres on the recording sheet.
If an image area ratio produced on the recording sheet becomes smaller, a frequency of refilling fresh toners in the developing unit may become smaller. Accordingly, a ratio of toners that remain in the developing unit for a longer time becomes greater and such toners may have received an agitation effect of a screw with a longer time, wherein the screw is provided in the developing unit to constantly agitate and transport toner in the developing unit.
Accordingly, a charging potential of such toners may increase, and thereby a toner amount to be adhered on the recording sheet may become smaller because of an imbalance between a latent image potential on the photoconductive member (i.e., image carrying member) and the charging potential of such toners.
Such condition may downgrade a developability of toner, so that an image density on the recording sheet may become lower. In order to compensate for such lower developability of the toner, fresh toners may be refilled into the developing unit, by which a toner ratio against carriers may increase in the developing unit. Accordingly, the toner concentration in the developing unit may increase.
If the toner concentration has increased beyond a specified level, unpreferable phenomenon, such as toner scattering and fogging, may occur. Such unpreferable phenomenon may be suppressed by setting a range for toner concentration.
However, even if the toner concentration in the developing unit is maintained within a predetermined range, a change of developer condition may downgrade a developability of the developer. If the developability of the developer downgrades, an image having lower density may be produced on the recording sheet.
When images of smaller image area are printed continuously with lower image density, the image forming apparatus may control the image density to a preferable level with less emphasis on toner concentration. In this case, the toner concentration may unfavorably exceed the range set for the toner concentration.
In another case, when images of smaller image area are printed continuously with lower image density, the image forming apparatus may control the toner concentration to a preferable level with less emphasis on the image density. In this case, an image density may become unfavorably lower.
FIG. 1 is a schematic view of a conventional image forming apparatus 100, which can be used as a printer, for example. The image forming apparatus 100 includes a photoconductive member 1, a charge roller 2, a laser diode unit 3, a developing roller 4, a transfer unit 5, a transport belt 6, a fixing unit 7, a density sensor 8, and a controller 10, as shown in FIG. 1.
The photoconductive member 1 serving as an image carrying member forms an electrostatic latent image and a toner image corresponding to the electrostatic latent image on its surface, as discussed below.
The charge roller 2 charges the surface of the photoconductive member 1. The laser diode unit 3 irradiates a light beam to the charged surface of the photoconductive member 1 to write the electrostatic latent image on the photoconductive member 1. The developing roller 4 develops the electrostatic latent image on the photoconductive member 1 as the toner image by adhering toner on the electrostatic latent image. The electrostatic latent image is developed as the toner image by using a potential difference between a developing bias voltage and the electrostatic latent image potential.
The transfer unit 5 transfers the toner image from the photoconductive member 1 to a recording sheet at a transfer position (i.e., nip position). The transport belt 6 transports the recording sheet to the transfer position (i.e., nip position) and fixing position. The fixing unit 7 fixes the toner image on the recording sheet. After the toner image is fixed on the recording sheet by the fixing unit 7, the recording sheet is transported to an ejection port (not shown), and ejected to an outside of the image forming apparatus 100. The density sensor 8 detects a toner density of a toner image pattern, which is transferred on the recording sheet.
The controller 10 controls the image forming apparatus 100 as a whole. The controller 10 includes a read only memory (ROM) and non-volatile random access memory (NVRAM), for example. The ROM stores controlling parameters such as developing bias voltage, charging bias voltage, transfer bias voltage, and laser diode power, for example.
The image forming apparatus 100 includes a tandem configuration for the photoconductive members 1 for producing magenta, cyan, yellow, and black images. The following processes are repeatedly conducted at each of the photoconductive members 1.
The charge roller 2 charges the photoconductive member 1, and then the laser diode unit 3 writes an electrostatic latent image on the photoconductive member 1 with a light beam. The developing roller 4 develops the electrostatic latent image on the photoconductive member 1 as a toner image, and then the transfer unit 5 transfers the toner image to a recording sheet.
The image forming apparatus 100 conducts an image density adjustment when images are produced on a predetermined number of recording sheets or when the image forming apparatus 100 is activated and warmed up, for example. Such image density adjustment is referred to as process control, hereinafter.
FIG. 2 is a flow chart for explaining an image density adjustment process (i.e., process control) for the image forming apparatus 100.
In step S101, the charge roller 2, developing roller 4, and transfer unit 5 are activated, and a bias voltage is applied to each unit and a motor is activated.
In step S102, a calibration of the density sensor 8 is activated at a predetermined timing.
As shown in FIG. 3, the density sensor 8 includes an infrared LED (light emitting diode) and a phototransistor, wherein a light beam emitted from the infrared LED reflects on a detection face (e.g., recording sheet) and the reflected light beam is received by the phototransistor.
For example, the infrared LED emits a first light beam generated with an electric current of PWM=128(=255/2) to the detection face, wherein PWM is pulse width modulation.
The phototransistor receives light reflected at the detection face and outputs an output signal. A central processing unit (not shown) receives the output signal of the phototransistor.
The calibration of the density sensor 8 is conducted by setting an output signal of the phototransistor, which corresponds to an output signal of a background area (i.e., no image area) of a recording sheet, as discussed below.
If the output signal of the phototransistor for the background area is greater than 4.1V(=4.0+0.1V), for example, the infrared LED emits a second light beam with another electric current of “PWM(2)=PWM(1)−(PWM(1)/2)” to the detection face. Then, the phototransistor receives a light reflected at the detection face (i.e., background area), and outputs another output signal. The central processing unit (not shown) receives the output signal of the phototransistor.
Hereinafter, PWM(n) means an electric current to be supplied to the infrared LED at each “n-th” time of lighting when the infrared LED emits a light beam for each “n-th” time.
For example, PWM(1) means an electric current to be supplied to the infrared LED when the infrared LED emits a first light beam, PWM(2) means an electric current to be supplied to the infrared LED when the infrared LED emits a second light beam after the first light beam, and PWM(3) means an electric current to be supplied to the infrared LED when the infrared LED emits a third light beam after the second light beam.
If the output signal of the phototransistor for background area is smaller than 3.9V(=4.0−0.1V) for the first light beam, the infrared LED emits a second light beam with another electric current of “PWM(2)=PWM(1)+(PWM(1)/2).” The phototransistor receives a light reflected at the detection face, and outputs another output signal. The central processing unit (not shown) receives the output signal of the phototransistor.
If the output signal of the phototransistor for background area is greater than 4.1V(=4.0+0.1V) for the second light beam, the infrared LED emits a third light beam with another electric current of “PWM(3)=PWM(2)−(PWM(1)/4).” The phototransistor receives a light reflected at the detection face (i.e., background area), and outputs another output signal. The central processing unit (not shown) receives the output signal of the phototransistor.
If the output signal of the phototransistor for background area is smaller than 3.9V(=4.0−0.1V) for the second light beam, the infrared LED emits a third light beam with another electric current of “PWM(3)=PWM(2)+(PWM(1)/4).” The phototransistor receives a light reflected at the detection face (i.e., the background area), and outputs another output signal. The central processing unit (not shown) receives the output signal of the phototransistor.
Such adjustment is repeated until the output signal of the phototransistor can be adjusted within a range of 4.0±0.1V for the background area of the recording sheet.
An output signal of the phototransistor, which is received lastly by the central processing unit (not shown) in the above-described process, is set as an electric current value for calibrating the density sensor 8. Such electric current value can be used for calibrating the density sensor 8 until a next new calibration is conducted for the density sensor 8.
After completing the calibration of the density sensor 8, a detection pattern is formed on the photoconductive member 1 to detect an image density of the detection pattern.
The detection pattern may have a rectangular shape having 20 mm in main scanning direction, and 15 mm in sub-scanning direction, for example.
In step S103, a charge bias voltage is set for a detection pattern PN(1). For example, the charge bias voltage for a detection pattern PN(1) is set to −300V. Hereinafter, each of the detection patterns are referred as PN(n), wherein n represents natural numbers.
After setting the charge bias voltage for the detection pattern PN(1), the laser diode unit 3 emits a laser beam to scan the charged photoconductive member 1 with a maximum value of laser diode power (e.g., 255) in step S104 to scan the rectangular pattern (e.g., 20 mm in main scanning direction, and 15 mm in sub-scanning direction).
When the detection pattern PN(1) comes to a position facing the developing roller 4, a developing bias voltage is set for the detection pattern PN(1) (e.g., −100V).
In steps S105 to S108, detection patterns PN(2) to PN(n) are formed on the photoconductive member 1 as similar to PN(1), wherein the detection patterns PN(1) to PN(n) are formed at a predetermined interval (e.g., 10 mm) in sub-scanning direction, for example. For example, n can be set to ten (n=10) to form ten detection patterns on the photoconductive member 1.
The ten detection patterns are applied with a bias voltage (e.g., 10 μA) by the transfer unit 5 at a transfer position to transfer the ten detection patterns to a recording sheet.
The recording sheet having the transferred detection patterns is transported by the transport belt 6, and the image density of the detection patterns are detected by the density sensor 8 in step S109.
The phototransistor (i.e., photosensor) in the density sensor 8 optically receives a light reflected on the recording sheet.
When no toner images is formed on the recording sheet, an output signal (i.e., electric current) of the phototransistor becomes greater, and when a toner image is formed on the recording sheet, an output signal (i.e., electric current) of the phototransistor becomes smaller because of an increased image density of the detection pattern. With such method, an image density on the recording sheet can be detected.
The output signal of the phototransistor is converted to image density data with a conversion equation for output signal/image density, which is stored in a ROM (read only memory). The image density data can be stored in a NVRAM (non-volatile random access memory) in the controller 10.
In such a process, a graph for detection patterns can be plotted with the image density data and the developing bias voltage, in which the vertical axis of the graph represents the image density, and the horizontal axis of the graph represents the developing bias voltage. A relationship of the image density and the developing bias voltage can be collinearly approximated with a least-square method. The gradient of the straight line can be stored in the NVRAM as a developing coefficient.
The controller 10 computes a developing bias voltage Vb from the developing coefficient and a target amount of toner adhesion. The target amount of toner adhesion can be set to 0.6 mg/cm2, for example.
For example, if the developing coefficient is 2.0 (mg/cm2/kV), the developing bias voltage Vb can be computed as Vb=(1/2.0(mg/cm2/kV))×1000=500V.
After computing the developing bias voltage Vb, the controller 10 refers the table storing image forming conditions in step S110, wherein the table is stored in the ROM.
In step S110, the controller 10 selects a condition, which is closest to the computed developing bias voltage (e.g., Vb=500V) from the table.
FIG. 4 shows an example table, which stores image forming conditions.
In step S111, a charge bias voltage, developing bias voltage, transfer bias voltage, and laser diode power are determined using the table, and such conditions are stored in the NVRAM (non-volatile random access memory) until a next process control is conducted.
In step S112, the controller 10 completes operations for the process control.
Such process control can be conducted concurrently for a yellow, cyan, magenta, and black toner image by detecting detection patterns formed by each color of toner. Accordingly, process controls for yellow, cyan, magenta, and black toner images are conducted in a parallel manner.
In the image forming apparatus 100, a relationship between the developing bias voltage Vb and a charge voltage Vc needs to be maintained at a predetermined level. Especially, when a two-component developer is used, a value computed by “Vb minus Vc” needs to be maintained at a predetermined level to prevent an adhesion of carriers onto the photoconductive member 1.
A change of the charge voltage Vc may cause a change of adhering capability of toner to the photoconductive member 1, a change of transfer-ability of toner image, and unfavorable image development on the photoconductive member 1 by an excessive transfer bias voltage.
Furthermore, a change of the developing coefficient, developing bias voltage Vb, or charge voltage Vc may cause a change of toner adhering capability to a high-lighted area.
The image forming apparatus 100 can detect a relationship between the toner adhering amount and the developing bias voltage as above-mentioned, by which an image density at a shadow area can be stabilized.
Furthermore, the image forming apparatus 100 can adjust a transfer bias voltage to cope with a change of transfer-ability of toners and unfavorable image development on the photoconductive member 1 by an excessive transfer bias voltage.
Furthermore, the image forming apparatus 100 can stabilize a toner adhering amount to the high-lighted area by using a predetermined laser diode power.
As above described, the image forming apparatus 100 includes an image density detector and a toner amount detector. The image density detector optically detects an image density of detection pattern, formed by a process of forming a latent image and toner image. Based on information detected by the toner amount detector, a toner concentration in the developing unit can be adjusted. With a combination of the image density detector and the toner amount detector, the image forming apparatus 100 may stably produce a higher quality image over time.
However, a toner concentration in the developing unit may increase beyond a specified range if only an output signal of the image density detector (e.g., optical detector) is adjusted within one target range of the image density. Such a drawback may occur depending on conditions and a type of developer to be used in the image forming apparatus 100.